Abstract:
For the first time upper part of the Cymodoce Zone sensu lato (mostly Askepta Subzone) of the Lower Kimmeridgian and the Mutabilis Zone of the Upper Kimmeridgian are described in detail from the Russian Platform. The most fully developed ammonite succession, which includes Boreal, Subboreal and Submediterranean faunal elements, is recognized in southern Tatarstan (Tarkhanovskaya Pristan and Memei sections). The lowermost assemblage, consisting of late representatives of Rasenia and Zonovia was found in the Memei section only (cf. Hantzpergue et al. 1998). Well above, numerous Submediterranean Crussoliceras occurrences were discovered in the upper part of the Cymodoce Zone of the whole area, whereas younger assemblages yielded Boreal Amoebites and/ or Subboreal Rasenioides followed by Aulacostephanoides with some addition of Aspidoceras and Orthaspidoceras above. The aulacostephanid genus Zenostephanus is recorded in the studied region for the first time. Significant lateral variability of the deposits of the Cymodoce and Mutabilis zones in thickness and completeness through a small distance in South Tatarstan along with other evidence suggests the strong influence of synsedimentary tectonics on sedimentation at the Early to Late Kim- meridgian transition of this region. Mass immigration of the Submediterranean ammonites Crussoliceras into the Subboreal Russian Sea resulted in their short-time dominance over a wide area. The Crussoliceras ammonites are recognized also in the condensed sections of the Moscow and Kaluga areas in the north, and their re-deposited records are known (along with uncommon Zenostephanus , Aulacostephanoides and Amoebites) from the basal phosphorite band of the Middle Volgian of the Unzha river (Kostroma area). This "Crussoliceras event", which is well-recognized in Submediterranean areas, as well as the subsequent expansion of Zenostephanus, both correspond to a sea-level high-stand, which resulted in faunal mixing of ammonites indicative of different bioprovinces. They provide key correlative levels which can be recognized around the Lower/Upper Kimmeridgian boundary, especially in the ecotone area of the Russian Sea. These levels include also occurrences of Crussoliceras, Rasenoiodes and rare Amoebites of the kitchini group in the Askepta Subzone, of Aulacostephanoides spp. along with Amoebites peregrinator and Zenostephanus sachsi in the lower part of the Mutabilis Subzone as well as of Aspidoceratidae and Aulacostephanoides in higher parts of this subzone, followed by Orthaspidoceras. At least six successive ammonite assemblages can be recognized through the studied part of the Kimmeridgian, providing interregional correlation with Submediterranean and Boreal sections.
